1. Field of the Invention
The present invention generally relates to a brake cooling mechanism for a four-wheeled vehicle, and particularly relates to the brake cooling mechanism which is suitable for a straddle type all-terrain four-wheeled drive vehicle.
2. Description of the Related Art
As a straddle type all-terrain four-wheeled drive vehicle, Japanese Laid-Open Patent Publication No. 2000-272315 discloses a vehicle in which a brake casing is integrated with a swing arm device for supporting rear wheels, a rear axle case is coupled to a rear end of the swing arm, a final reduction gear for rear wheels is housed inside the rear axle case, and a wet multiple-disk braking device is housed inside the brake casing.
In order to lubricate and cool down the brake device and the final reduction gear, an oil chamber is provided in a part inside the swing arm device, which is a different part from the brake device, and the lubricant stored in the oil chamber is employed for lubricating and cooling down the brake device and the final reduction gear.
On the other hand, Japanese Laid-Open Patent Publication No. 59-130791 discloses a three-wheeled vehicle, as another piece of prior art, in which a wet multiple-disk braking device, composed of a friction plate, a separator plate, etc., is mounted between a brake center fixed to a ring gear of a differential device and an inner surface of a case of the differential device.
The friction plate of the braking device, etc. is cooled down by employing a lubricant stored inside the casing of the differential device.
In the aforementioned pieces of conventional art, the brake device is cooled down on the basis of the cooling effect by the lubricant which is commonly used in the final reduction gear, and is based upon the radiation effect (releasing effect) of the friction heat from a surface of the casing by air which is gained while the vehicle is running. However, in the conventional arts, a means or artifice to positively promote the radiation of heat from the surface of the casing is not provided.
According to the former piece of prior art, the oil chamber is mounted separately from a brake chamber and a final reduction gear chamber. Therefore, the path to allow the lubricant to circulate becomes complex.
On the other hand, according to the latter piece of prior art, inside a casing, accommodating a large bevel gear (ring gear) having a large outer diameter, of the final reduction gear, there is mounted the brake device having an outer diameter greater than the large outer diameter of the large bevel gear. Therefore, the casing of the final reduction gear becomes large, and the minimum height of the casing relative to the ground level becomes low, thus unsuitable for the straddle type four-wheeled drive vehicle. In addition, in order to increase the amount of the lubricant, it is necessary to downwardly extend a lower part of the casing of the final reduction gear. That is, the amount of the lubricant is limited by the construction.